This paper describes the design and implementation of a network virtualization substrate ( ) for effective virtualization of wireless resources in cellular networks. Virtualization fosters the realization of several interesting deployment scenarios such as customized virtual networks, virtual services, and widearea corporate networks, with diverse performance objectives. In virtualizing a base station's uplink and downlink resources into slices, meets three key requirements-isolation, customization, and efficient resource utilization-using two novel features: 1) introduces a provably optimal slice scheduler that allows existence of slices with bandwidth-based and resource-based reservations simultaneously; and 2) includes a generic framework for efficiently enabling customized flow scheduling within the base station on a per-slice basis. Through a prototype implementation and detailed evaluation on a WiMAX testbed, we demonstrate the efficacy of . For instance, we show for both downlink and uplink directions that can run different flow schedulers in different slices, run different slices simultaneously with different types of reservations, and perform slice-specific application optimizations for providing customized services.

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This paper examines the costs and potential benefits of long-term prefetching for content distribution. In contrast with traditional short-term prefetching, in which caches use recent access history to predict and prefetch objects likely to be referenced in the near future, long-term prefetching uses long-term steadystate object access rates and update frequencies to identify objects to replicate to content distribution locations. Compared to demand caching, long-term prefetching increases network bandwidth and disk space costs but may benefit a system by improving hit rates. Using analytic models and trace-based simulations, we examine several algorithms for selecting objects for long-term prefetching. We find that although the web's Zipf-like object popularities makes it challenging to prefetch enough objects to significantly improve hit rates, systems can achieve significant benefits at modest costs by focusing their attention on long-lived objects.

Adaptive transmit power control in 802.11 Wireless LANs (WLANs) on a per-link basis helps increase network capacity and improves battery life of Wifi-enabled mobile devices. However, it faces the following challenges: (1) it can exacerbate receiver-side interference and asymmetric channel access, (2) it can incorrectly lead to lowering the data rate of a link, (3) mobility-induced channel variations at short timescales make detecting and avoiding these problems more complex. Despite significant research in rate and power control, state of the art solutions lack comprehensive techniques to address the above problems.In this paper, we design and implement Symphony-a Synchronous Two-phase Rate and Power control system, whose agility in adaptation enables us to systematically address the three problems, while maximizing the benefits of power control on a per-link basis. We implement Symphony in the Linux MadWifi driver, and show that it can be realized on hardware that supports transmit power control with no modifications to the 802.11 MAC, thereby fostering immediate deployability. Our extensive experimental evaluation on a real testbed in an office environment demonstrates that Symphony (1) enables up to 80% of the clients in 3 different cells to settle at 50% to 94% lower transmit power than a percell power control solution, (2) increases network throughput by up to 50% across realistic deployment scenarios, (3) improves the throughput of asymmetry-affected links by 300%, and (4) opportunistically reduces the transmit power of mobile clients running VOIP calls by up to 97%, while causing minimum impact on voice quality.

Most packet processing applications receive and process multiple types of packets. Today, the processors available within packet processing systems are allocated to packet types at design time. In this paper, we explore the benefits and challenges of adapting allocations of processors to packet types in packet processing systems. We demonstrate that, for all the applications and traces considered, run-time adaptation can reduce energy consumption by 70-80% and processor provisioning level by 40-50%. The adaptation benefits are maximized if processor allocations can be adapted at fine timescales and if the total available processing power can be allocated to packet types in small granularities. We show that, of these two factors, allocating processing power to packet types in small granularity is more important-if the allocation granularity is large, then even a very fine adaptation time-scale yields meager benefits.

Switched beam antennas are an attractive extension to indoor wireless LANs due to their increased signal gain in a chosen direction; the gain can be exploited for improving wireless link quality, node localization and increasing spatial reuse. However, indoor environments are susceptible to multipath reflections that may reduce the degree of directionality of the antennas. To this end, in this paper, we address the following questions that have not been explored well in the open literature: how directional in reality is a beam with a switched beam antenna in a reflection-rich environment, and what are the implications of the observed directionality on spatial reuse and node localization? And how does the directionality get affected with the characteristics of a beam such as main and side lobe width, and front to side lobe ratio? We present results of measurements in a real office setting with a switched beam antenna built out of an 8-element phase array.

Several research works have argued that adaptive beamforming has the potential to realize the high spectral efficiency requirements of next-generation wireless standards, and is especially well-suited for multipath-rich environments such as indoors. Most works have been limited to theory; few works in literature address the practical benefits and realizability of adaptive beamforming. In this paper, we design and implement the first indoor WLAN beamforming system with multi-element array antennas and software radio platforms, that forms a testbed for exploration of practical benefits of beamforming, and evaluation of algorithms for efficient beamforming in diverse environments. In the process of building the system, we identify and address several challenges with practical beamforming that are often ignored in theoretical works. Most importantly, channel estimation for forming the best beam to a user is hindered by oscillator drifts on the transmitter and receiver side that introduce hard-to-isolate phase and frequency offsets from the estimated channel coefficients. We describe these issues and incorporate novel solutions in our system to address them without requiring hardware modifications. We use the system to demonstrate the realizable benefits of adaptive beamforming in a typical indoor office environment.

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